Bactrites

Bactrites, genus of extinct cephalopods (animals related to the modern squid, octopus, and nautilus) found as fossils in marine rocks from the Devonian to the Permian periods (between 408 and 245 million years ago). Some authorities have identified specimens dating back to the Silurian Period (beginning 438 million years ago), but their classification is uncertain. The shell consists of a linear series of chambers, each successively occupied by the body of the animal. Bactrites fed on animals it caught in its tentacles. It is possible that Bactrites gave rise to more advanced cephalopods of later geologic periods, notably the ammonoids and the belemnoids.

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The Ammonoidea are usually considered to have evolved from Devonian (416 million to 359.2 million years ago) straight-shelled forms (Bactrites) with certain nautiloid traits. Coiled ammonites appeared in the Late Devonian Epoch (Goniatites). The subclass became extinct in the Cretaceous.